The "A-B-A-B" Design Is a Slight Variation on the A-B-A

The "A-B-A-B" design is a slight variation on the A-B-A design, in which treatment is returned at the end of the experiment. Under what conditions would this be most desirable?

A) if the researcher is still not sure whether the treatment was effective
B) if the treatment has some beneficial therapeutic effect
C) if the treatment did not produce the desired effect the first time it was presented
D) if the participant enjoyed the treatment